[chiralSPO (OP)]

I have long been fascinated by the idea of habitable "rogue" or "orphan" planets that are not associated with any star, just flying through interstellar (even intergalactic?) space, but are kept at reasonable temperatures purely by fission of unstable isotopes within the planet. The recent detection of the neutron star collision has re-ignited my interest in the subject.

The ability to detect exoplanets has only been around for a few decades, but we have already established that there are probably more planets in orbit around stars than there are stars (most stars have at least one orbiting planet). But as far as I know, all of the methods we use find planets indirectly by directly observing the star. Rogue planets are therefore invisible to these techniques. And direct observation of a rocky planet that is not illuminated by a very nearby star is going to be extremely difficult (we can't even see whatever planet they think is past Pluto). But rogue planets in general might be quite common. There are enough binary (and ternary and quaternary) star systems out there, or solar systems with massive gas giants, that planets and moons must be thrown out all the time.

The particular case I am interested in is this: we now know (it had been theorized, but now is confirmed) that neutron star collisions generate and eject massive amounts of heavy elements (stable and unstable isotopes). Could a high local concentration of heavy elements lead to a small rocky planet that is highly enriched in elements heavier than nickel?

Most of the "short-lived" radioactive elements would be gone within say 50,000 years, and then you would probably be left with the same sorts of decay chains that we see on earth, just with higher abundances. If the abundances are just right (no idea what the range would be, but it would depend on surface area, albedo, atmosphere etc.) the planet could maintain a quasi-equilibrium between heat released through fission, and heat radiated to space by blackbody. Such planets would probably just cool monotonically from a maximum temperature (from aggregation and short-lived isotopes) down to space temp several (potentially many, many) billions of years later--so as long as it started out "too hot" the planet would still certainly be at a "habitable" temperature at some point during this process.

Chemistry (and biology!) on such planets would be extremely interesting. Of course, the same rules would apply, but have a natural world filled with platinum group metals and heavy halogens would allow for some very different natural chemical processes than we see here on earth.

If the planet were small enough (and warm enough) then it won't be able to hold onto volatiles like H₂, He, CH₄, NH₃, H₂O, Ne, N₂, CO, CO₂... It could be of an intermediate size where H₂ and He won't stick (most of what it would likely come across as it traverses the vast expanses). Or it could be large enough to hold onto everything it comes across, and won't probably turn into a gas giant or even seed a new star... The planet might also be producing quite a bit of helium to maintain its temp, so if it can hold on to He then its mass won't decrease, but the atmospheric pressure will increase and the fraction of He in it will increase....

Such planets would also probably be quite dense, and could have significantly higher surface gravity, with interesting implications for mechanics (geological and biological).

Anyway, I'm rambling now. Thoughts?

[evan_au]

There have been bacteria found on Earth, apparently living on the products of radioactive decay.
https://www.princeton.edu/news/2006/10/20/two-miles-underground-strange-bacteria-are-found-thriving

And there are some very radiation-hardened bacteria out there.
https://en.wikipedia.org/wiki/Deinococcus_radiodurans

Back when Dark Matter was thought to be primarily made up of rogue planets and black holes, several searches were made using gravitational microlensing - this showed that there weren't enough of them to account for Dark Matter.
These searches have continued, using modern techniques. They are best at detecting brown dwarfs and black holes, but some largish planets have been discovered.
https://en.wikipedia.org/wiki/Gravitational_microlensing#Detection_of_extrasolar_planets

I suspect that any rogue planet with water on the surface would be like Europa - a thick layer of ice (facing the 2.7K CMBR), with any liquid water heated from underneath.

[Colin2B]

Mmm, bit like underfloor heating.
Assuming high levels of mutation some bacteria might develop.
Would surface screen be enough to prevent damage to tissue or would adaptations, such as insects which dont grow all the time, be the best lifeform.
I like the idea.

[chiralSPO (OP)]

I'm not really sure how much "harmful" radiation there would be at the surface...

Given the rate of energy release required to maintain surface temp of a planet-sized sphere at 274 K  (just above pure water mp), there would need to be a lot of decay going on. But presumably such planets would have enough heat for the materials to separate out (differentiate). I think it is reasonable to think that most of the radioactive elements, like uranium (19ish g/cm³, depending on isotope ratio) would be concentrated in the depths, while less dense elements like tin (7.3 g/cm³) or even lead (11.3 g/cm³) might stay near the surface. If the non-radioactive elements do a better job or screening neutrons and γ-rays than our silicate minerals, then there might not be as much surface radiation as one would expect...

But I don't have a good idea for what kinds of geological processes might be involved (nuclear volcanos?). There could also well be plenty of radioactivity from radon, or other lighter elements... We also have no shortage of iron and nickel in the crust despite most of it having concentrated in our core, so similar processes might bring significant amounts of core-forming materials to the surface.

Of course, it is also entirely possible that with the different elements around, life might not use DNA (or any other familiar biochemistry)...

[chiralSPO (OP)]

There have been bacteria found on Earth, apparently living on the products of radioactive decay.
https://www.princeton.edu/news/2006/10/20/two-miles-underground-strange-bacteria-are-found-thriving

And there are so very radiation-hardened bacteria out there.
https://en.wikipedia.org/wiki/Deinococcus_radiodurans

Back when Dark Matter was thought to be primarily made up of rogue planets and black holes, several searches were made using gravitational microlensing - this showed that there weren't enough of them to account for Dark Matter.
These searches have continued, using modern techniques. They are best at detecting brown dwarfs and black holes, but some largish planets have been discovered.
https://en.wikipedia.org/wiki/Gravitational_microlensing#Detection_of_extrasolar_planets

I suspect that any rogue planet with water on the surface would be like Europa - a thick layer of ice (facing the 2.7K CMBR), with any liquid water heated from underneath.

Thanks for the links!

I agree that something like Europa would be a likely scenario. Maintaining the energy flux of a very cold surface while allowing for liquid water deeper down would certainly require less radioactive decay, and a state like that could be much longer lived than warmer surfaces (blackbody radiation power goes as T⁴, if I recall correctly, so if the power from decay falls by 10% 274 K would fall to 180 K, while 5 K would only fall to 3 K.)

[Janus]

Let's use Earth as a comparison, it is estimated that the radioactivity of the Earth produces 1/2 or 2e13 watts worth of the Earth's internal heat.  The surface of the Earth receives 1.8e17 watts from the Sun.  If we want to make up this energy inflow with interior radioactivity, you would need to increases the natural radioactivity of the Earth by ~9000 times.

[chiralSPO (OP)]

Let's use Earth as a comparison, it is estimated that the radioactivity of the Earth produces 1/2 or 2e13 watts worth of the Earth's internal heat.  The surface of the Earth receives 1.8e17 watts from the Sun.  If we want to make up this energy inflow with interior radioactivity, you would need to increases the natural radioactivity of the Earth by ~9000 times.

Yes, that's about in line with what I calculated for a sphere with the volume of the Earth. This will change somewhat for different sizes, and will also depend on the atmosphere.

[chiralSPO (OP)]

I recently came across this video discussing theories on origins of life:

around 2:35 they discuss the possibility of geologically dead rogue planets being hospitable in the primordial universe, when the cbmr was sufficiently blue (hot) to support "habitable" conditions on the surfaces. It is a very interesting thought that I would never have considered, and I figured it was worth sharing here.